Low noise process for breaking pavement which relies upon reflected tensile pulses to fracture the pavement

ABSTRACT

A method for breaking pavement which, in general, includes the steps of creating a continuous or discontinuous free boundary surface about and defining the area of the surface to be broken or fragmented, and then imparting to this defined area high intensity, short duration compressive wave pulses of such number, location and intensity that these compressive waves when reflected at the free surface as tensile wave pulses will exceed the tensile strength of the pavement and cause the latter to break up so as to produce fragments of a desired size for subsequent removal.

United States Patent Inventor Philip J. Anderson Deerfield, Ill.

Appl. No. 846,032

Filed July 30, 1969 Patented Oct. 19, 1971 Assignee Institute of Gas Technology Chicago, Ill.

LOW NOISE PROCESS FOR BREAKING PAVEMENT WHICH RELIES UPON REFLECTED TENSILE PULSES T0 FRACTURE THE PAVEMENT 8 Claims, 19 Drawing Figs.

U.S. CL 299/14, 299/17 Int. CL E01c 23/09 Field of Search 299/14, 36, 37; 175/56, 67

[56] References Cited UNITED STATES PATENTS 2,308,517 1/1943 Konnerth 299/ 14 3,232,669 2/1963 Bodine 175/56 X 3,302,720 2/1967 Brandon 175/56 X 3,443,051 5/1969 Puschner 299/140 X Primary Examiner-Ernest R. Purser Attorney-Dominik, Knechtel & Godula ABSTRACT: A method for breaking pavement which, in general, includes the steps of creating a continuous or discontinuous free boundary surface about and defining the area of the surface to be broken or fragmented, and then imparting to this defined area high intensity, short duration compressive wave pulses of such number, location and intensity that these compressive waves when reflected at the free surface as tensile wave pulses will exceed the tensile strength of the pavement and cause the latter to break up so as to produce fragments of a desired size for subsequent removal.

PATENTEUUBT 19 IHTI SHEET 2 OF 3 PAVEMENT COMPRESS/ VE PULSE FIG IQ FIG IQ FIG IA TENS/LE PULSE FIG 72 FIG 7; FIG LE FIG. 8

/N VE N TOR Philip J. Anderson 17% fiazl am ATT'YS r PATENTEflnm 19 ml SHEET 30F 3 FIG. [0'

C OMPRE S S VE STRAIN PULSE FRACTURE a DEVELOPS -l TENS/LE STRAIN PULSE IN VENTOR Philip J. Anderson lam-, Mm

ATTYS LOW NOISE PROCESS FOR BREAKING PAVEMENT: WHICH RELIES UPON REFLECTED TENSILE PULSES TO FRACTURE THE PAVEMENT This invention relates to an efficient, low noise method of fragmenting pavement of concrete, asphalt or other similar materials, within a predefined area, to facilitate its removal. The term "pavement" is used herein as intended to include roads, streets, sidewalks and other slab-type surfaces of concrete, asphalt, or combinations of the two, or other similar materials, with or without reinforcing rods or mesh therein.

The apparatus presently most commonly used for breaking solid materials such as those mentioned above is the pneu matic hammer. While extremely efiective, the use of pneumatic hammers often causes public complaints when they are used in densely populated areas. The reason is that both the pneumatic hammer itself and the air compressor that supplies high-pressure air to the hammer produce irritating noise of high intensity. This noise is particularly objectionable in areas where hospitals and schools are located. This noise also excludes, or at least restricts the scheduling of excavation operations at night which, in many cases, such as pavement removal for repairing or laying underground cables and/or pipes, would reduce the down time and the inconvenience to the public.

Recently, considerable effort has been and is being made to develop a low noise method for breaking solid materials, particularly pavement. Electric and gas utility companies are especially interested in the development of such a method because of the need to remove pavement to lay new underground cables and pipes, or to repair existing cables and pipes.

One low noise method recently developed involves the use of a plasma torch which actually melts a perimeter kerf around the pavement that is to be removed. Improved torch designs were developed during the investigation of the plasma torch as an effective low noise device for pavement removal, however, its use was substantially abandoned because of the problem of disposing of the molten material in the kerf, that is, the melted concrete, which prevented making cuts of sufficient depth to penetrate pavement. Accordingly, industry is still seeking an effective operable, low noise method and apparatus for breaking solid material.

ln U.S. Pat. No. 2,859,952, there is disclosed a method of mining using high-frequency magnetic energy, to fracture and comminute the ore as mined so that the latter will not be in large, unwieldy boulders, but rather in relatively small fragments. The fracture operation achieved results from producing localized expansion so as to subject adjacent areas of the rock containing the ore to tensile stresses beyond the fracture strain thereof.

The method and apparatus of this patent may be classified as a low noise method of breaking solid material, however, its use is generally restricted to mining-type operations where the ore or material has a metallic content responsive to high frequency magnetic radiation. The ore or material thereafter can be more easily broken up into small fragments. The large amount of energy. approximately 25 kilowatts, and the apparatus required to generate this amount of energy make the disclosed method less than desirable.

In U.S. Pat. No. 3,219,280, there is disclosed another method and apparatus for splitting nonmetallic brittle materials using ultrahigh frequency oscillations. The apparatus of this patent likewise generates or produces exceptionally high internal tensions in the interior of the stone, to fracture it so that the stone thereafter can be easily broken up into small fragments.

More particularly, the apparatus of this last-mentioned patent comprises a radiating element in the form of an injector having two relatively perpendicularly polarized radiators each radiating substantially into the same local space. ln operation, the injector is introduced into a hole drilled into the stone, thus the ultrahigh frequency oscillations are locally concentrated in the interior of the stone at the area of the injector so 75 drawings in which:

that the .stone cracks and thereafter can be broken up into smaller pieces.

In a copending U.S. Pat. application, Ser. No. 817,783, filed Apr. 21, 1969, there is disclosed another low noise method for breaking solid material which involves coupling microwave energy into the solid materials such as concrete or rock to thermally induce stresses leading to failure in compression or tension, in a known and predictable manner. The apparatus for coupling the microwave energy into the solid material is adapted to generate independent hean'ng patterns by using, for example, at least two or more microwave applicator horns that are spaced apart from each other. The applicator horn or horns for generating the heat patterns are positioned on or slightly above the solid material to be fractured. The spaced heat patterns or zones produced cause the heated material to grow" or expand, due to its coefficient of heat expansion, so

as to place high tensile forces or stresses on the unheated material between the heat patterns. These forces or stresses cause failure to occur between the heat patterns, and ultimately across the heat patterns themselves. In this manner, a substantially straight crack along a desired course between adjacent heat patterns can be produced. The crack that is generated is a crack, which extends through the solid material.

The method and apparatus of this copending application is extremely effective for breaking solid materials such as pavement, thus creating cracks which may define an area of pavement to be removed. However, it is found that the cracks formed in the pavement are not vertical, as would be formed, for example, with a saw cut, but are rather jagged cracks. As a result, the area of pavement to be removed is still keyed with the adjacent pavement so that, in most cases, this area to be removed must subsequently be fragmented. The disclosed methods and apparatus are not particularly effective or practical for use in fragmenfing the pavement and thus some other means are normally employed. If these means are conven tional means such as the irritating noise-producing pneumatic hammer, the advantages gained by the use of the accompanying low noise method and apparatus are lost.

Accordingly, it is an object of the present invention to provide improved low noise methods of pavement removal.

More particularly, it is an object to provide an improved low noise method of fragmenting pavement, within a defined area, to facilitate its removal.

Another object is to provide an improved low noise method of the above-described type which generally is far more efficient in operation than most low efiiciency, high noise methods presently used to fragment such type surfaces.

-A still further object is to provide an efficient, low noise method of fragmenting such type surfaces which is of such a low noise level that operations can be conducted at night, with little, if any, objection from the public.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The above objectives are accomplished with the method of the present invention which, in general, includes the steps of creating a continuous or discontinuous free boundary surface about and defining the area of the surface to be broken or fragmented,-and then imparting to this defined area high intensity, short duration compressive wave pulses of such number, location and intensity that these compressive waves when reflected at the free surface as tensile wave pulses will exceed the tensile strength of the pavement and cause the latter to break up so as to produce fragments of a desired size for subsequent removal.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others thereof, which will be exemplified in the method hereinafter disclosed, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying FIG. 1 is a pictorial-type view generally illustrating apparatus exemplary of the type which can be used to carry out the method of the present invention, to fracture solid material such as pavement:

FIG. 2 is a front plan view of a multihom applicator;

FIG. 3 is a side plan view of the multihom applicator of FIG. 2;

FIG. 4 is a view generally illustrating the heat patterns or zones generated by the multihom applicator of FIGS. 2 and 3, and the crack resulting between them;

FIG. 5 is a section view of a single-stage intensifier for producing a high velocity, pulsed, water jet;

FIG. 6 is a graph illustrating the impact pressure and static pressure in terms of jet velocity;

FIGS. 7A-F are views generally illustrating the reflection of a compressive strain pulse in the pavement;

FIG. 8 is a top plan view generally illustrating the crushed crater area formed in the pavement of the high velocity, pulsed, water jet;

FIG. 9 is a cross-sectional view taken along lines 9-9 of FIG. 8;

FIG. 10 is a top plan view generally illustrating the radial cracks formed in the pavement; and

FIGS. llA-D are views generally illustrating the tensile fracture resulting from the reflection of a compressive strain pulse.

Similar reference characters refer to similar parts throughout the several views of the drawings.

The method of the present invention is an economical, low noise, two step method for breaking pavement. The first step consists of making a crack to outline the area within which the pavement is to be broken. This crack, as more fully defined below, functions as a free surface for reflecting compressive shock pulses. If the area is already outlined by, for example, its own boundaries and one or more expansion joints or the like, this step, of course, can be eliminated. The second step is the fragmenting of this pavement.

The first-mentioned step can be performed in a number of different ways, including but not limited to sawing, using an electron beam, a laser, a forced flame or a plasma torch, or by jet piercing, erosion cutting or abrasion cutting the pavement. In accordance with a preferred and specifically disclosed embodiment of this invention, a microwave applicator of the type disclosed in the above-mentioned copending U.S. Pat. application, Ser. No. 817,783, filed Apr. 2i, 1969, advantageously is used to create the perimeter crack defining or outlining the area of pavement to be removed. In the disclosed embodiment of the invention, the compressive shock pulse induced into the pavement to perform the second step is induced by means of a water jet which is adapted to produce a high velocity, pulsed, jet of water. The second step, however, can be performed using an electrospark, an explosive, electric disintegration, an electric arc, explosive pellets, a high-frequency electric discharge, an electric heater, or by induction heating, to mention but a few of the many different methods or apparatus which can be used. Those mentioned methods and apparatus which are not inherently "low noise" types generally can be muffled, at least to the extent that they are far less noisy than a pneumatic hammer.

The specifically described method of the invention contemplates directing microwave energy against the pavement through applicator horns which are positioned in a fashion such as to generate a substantially straight crack in the pavement to create a free surface. The crack results from the high tensile forces or stresses imposed on the unheated material between the spaced heat patterns or zones produced by microwave energy. When the area of pavement to be removed has been outlined by these cracks, or free surfaces, the high velocity, pulsed, water jet is used to produce a pulse of water which is caused to strike the pavement to create a compressive shock pulse. As the pulse radiates hemispherically from the impact point, it causes tangential tensile stresses which crack the pavement. These cracks radiate out from the crater formed at the point of impact. When the-pulse or wave is reflected at the free surfaces, it changes its form fromcompressive to tensile. The pavement breaks as a result of the intensity of the reflected wave exceeding its tensile strength. The presence of several free surfaces plus their nonsymmetrical orientation to the incident wave results in a complex fracture pattern which is advantageous with respect to removal of the fragments produced as they do not tend to interlock.

This particular apparatus and method of fragmenting the pavement is characterized by its low noise and high efficiency. A high velocity slug of water is an efficient means of producing the compressive pulse in the pavement, and the use of the same is preferred, because of its low noise and high emciency. The energy in the slug of water can be comparable to that produced by an equivalent weight of chemical explosive, but it does not involve the objectionable noise of the airborne detonation wave associated with explosives. The mechanism of fragmentation is also efficient as it is the result of exceeding tensile rather than compressive strengths; the tensile strength of concrete, for example, being as little as one-fiftieth of its compressive strength. An energy density of approximately 0.156 Btu/lb. is required for fragmentation of concrete by these processes. This level, for pavement slab sizes involved in normal breaking operations, can be generated in time periods measured in seconds. Also, the size and distribution of the fragments produced can be varied depending on the intensity of the energy pulse and the location of the impact point with respect to the free surfaces.

More specifically, in FIGS. 2 and 3 of the drawings, there is illustrated a multihom applicator 10 of the type disclosed in the above-mentioned copending U.S. Pat. application. Reference may be made to this copending application for a complete description of the construction and operation of the applicator 10, however, generally, it has four (as illustrated) applicator horns 11-14 which when linearly aligned are capable of generating or producing a controlled or predictable crack, along a desired course, with little or no attendant noise.

The multihom applicator 10 is constructed of two identical modular frames 15 and 16 which are adapted to be affixed together to form an integrated unit. These modular frames 15 and 16 also support the microwave components, including microwave power generating devices, such as magnetrons, the applicator horns lll4 and the waveguide system for coupling the output of the microwave power generating devices to the applicator horns. A microwave power supply 35 (FIG. 1) is coupled to the microwave power-generating devices by means of coaxial cables 36 and 37 which preferably are heavy-duty high-voltage cables. A liquid-type heat exchanger 38 also is provided, and the latter is coupled to the microwave power-generating devices by means of the water hoses 39. As can be seen in FIG. 1, in the illustrated embodiment, both the microwave power supply 35 and the heat exchanger 38 are housed in a motorized van 40 which also is adapted to receive the multihom applicator 10, so that the system can be easily transported from one location to another. The van 40 also is advantageously provided with a special transmission having a power takeoff gear assembly 41 which is drivingly coupled with a generator 42 that generates the current for powering the power supply 35 of the microwave system. In this manner, a completely self-contained unit is provided so that no outside source of power is required, and the multihorn applicator 10 is usable in any remote location.

The applicator horns 11-14 are rectangular-shaped horns, and each of them preferably has a skirt-type shield 46 of metal screening affixed about the lower ends thereof, for preventing stray radiation. These shields 46 also preferably are weighted about the peripheral edges thereof so that the edges sit on and conform to the surface of the concrete pavement. Vent holes are provided in the applicator horns to permit the escape of moisture driven out of the concrete. A window or plug of Teflon or other low-loss material which is transparent to microwaves is affixed within each of the applicator horns to prevent extraneous material such as moisture, concrete dust and chips from entering and damaging the waveguide system and/or the microwave power generating devices. In operation, the applicator horns 11-14 preferably are positioned on or just slightly above the surface of the pavement. Accordingly, an adjustable assembly 49 is incorporated into each of the modular frames 15 and 16, for raising and lowering the applicator horns to position them. The adjustable assemblies 49 also are adapted to horizontally or laterally adjustably position the applicator horns 11-14 so that the lateral spacing between them can be varied.

The applicator horns 11-14 are spaced apart from each other so as to generate independent heat patterns or zones in the concrete. In order to produce a substantially straight crack along a desired course, the applicator horns are laterally aligned, and are spaced on or slightly above the concrete to be fractured. The microwave energy admitted by the applicator horns l l- 14 is generally equally distributed. The spaced heat patterns produced by the applicator horns cause the heated material to expand, due to its coefficient of heat expansion, so as to place high tensile forces or stresses on the unheated material between the heat patterns. These heat patterns generally appear as discolored areas on the concrete, as represented by the reference numerals 50 in FIG. 4. As soon as a crack indicated by the reference numeral 51 develops, it is usually detectable by a damp or moist line which forms on the surface of the pavement, along the path of the crack.

The applicator is operated in the above-described fashion, until the area of pavement to be removed is defined or outlined by the cracks produced therein. The cracks create or form a free surface, for reflecting the compressive pulses or waves induced in the pavement, by means of a high velocity pulse of water, in the manner described below.

In FIG. 1, there is illustrated a water jet apparatus 60 which can be used to generate this high velocity pulse of water. However, as indicated above, it also can be generated in a number of other manners also. The water jet apparatus 60 can include a single-stage intensifier 61 of the type shown in FIG. 5, which may be, for example, a single stage intensifier of the type sold by Exotech Incorporated. An intensifier of this type generally includes a high-pressure chamber 62 having a discharge nozzle 63 at its one end, a low-pressure chamber 64, and a piston 65. In operation, hydraulic fluid under high pressure is released by a quick release valve (not shown) and coupled through an inlet 66 into a chamber 67 behind the head 68 on the piston 65. This activates the piston 65 to force water contained in the high-pressure chamber 62 out through the discharge nozzle 63. As the piston 65 moves, it also displaces hydraulic fluid from the low-pressure chamber 64, through the outlet 69. This hydraulic fluid is pressurized and flows back through the same outlet 69, to return the piston 65 to its original position. The piston 65, in turn, displaces the hydraulie fluid in the chamber 67 behind its head 68. This hydraulic fluid is again pressurized and released to again activate the piston 65. Water is coupled to the high-pressure chamber 62 through the inlet 70.

The intensifier 61 can be transported and supported in operative position, by means of a trailer 75 adapted to be coupled to and pulled by the van 40. This trailer 75 also is adapted to carry the equipment such as a water supply 76, a high pressure pump 77 and a hydraulic accumulator 78, for operating the intensifier 61. The equipment furthermore is operated by the power-generating equipment in the van.

The discharge nozzle 63 of the intensifier 61 preferably is of a design to produce a high velocity water jet in the form of a coherent liquid cylinder, of high velocities for several inches. As this pulse or jet of water strikes the pavement, part of the energy is released as a short duration high intensity compressive pulse or wave which moves hemispherically out into the pavement. The pressure at impact for a water jet slug impacting normally on the surface is given by the water hammer equation:

P;=pcv/g where:

p= density of the fluid, lbs/ft. c compression wave velocity in the liquid (assumed to be 5250 ftJsec.)

v velocity of the jet, ft./sec.

g= gravitational constant, ft./sec.

For jet velocities above 1500 ft./sec. the intensity of this initial pulse, as can be seen in the graph of FIG. 6, exceeds the compressive strength of the pavement. The remaining energy of the pulse of water is released more slowly at a lower intensity, as indicated by the static pressure line. As no side restraints to the flow of the pulse exists, the liquid from the pulse flows out across the surface or into the radial cracks in the pavement at a high speed. This flow may produce shearing or tearing of the surface, as well as widening and propagation of the radial cracks. Due to the short duration and the high intensity of the impact compressive pressure pulse, the water jet behaves as a chemical charge exploded on the surface. Unlike a chemical explosive, however, the water pulse does not produce the airborne detonation wave created by the collapse of the partial vacuum from the burning process.

The initial impact of the water slug produces a high intensity, short duration pulse. This pulse is a compressive shock wave which radiates hemispherically from the impact point. The increase in intensity of this wave with the distance traveled from the impact point has been shown to be an exponential function:

where:

A intensity of shock pulse, p.s.f.

X travel distance, ft.

e base for natural logarithms C constant, lbsf/ft.

a absorption constant, ft.

Thus, the intensity of the shock wave at any distance from the impact point is determined by the intensity of the shock pulse generated at the shock point, the distance traveled by the shock pulse and the propagation characteristics of the pavement material.

At any free surface, the compressive shock pulse is reflected as a tensile pulse. The process of reflection is shown in FIGS. 7A-7F. The free surfaces are the perimeter cracks formed, for example, in the above-described manner using the microwave applicator l0, and the base of the pavement slab to be broken. In the process of reflecting some of the pulse, energy is transmitted across the free surface. The effectiveness of the free surfaces as reflectors is a function of the angle of incidence of the impinging pulse and the mechanical impedance of the materials on each side of the free surface. The more nearly normal to the surface, the compressive pulse impingement and the greater difference the mechanical impedances, the greater the energy in the reflected pulse. The perimeter crack and base of the slab are effective reflectors. In FIG. 7, the free surface 80 is vertical. The horizontal axis is the direction of propagation of the pulse and the value of zero stress. Above the horizontal axis is increasing tensile stress and below the horizontal axis compressive stress. It can be seen that the maximum intensity of the resultant pulse, the sum of the incident and reflected pulses, does not develop at the free surface 80, but rather at some distance back in the pavement. The maximum tensile stress is developed at a distance from the free surface 80 equal to half the fall length of the incident compressive pulse. The fall length is the product of time required for the amplitude of the compressive pulse to fall to one-half its maximum value and the velocity of propagation. Before it reaches this maximum, the magnitude of the tensile stress developed during reflection depends primarily on the shape of the decaying portion of the incident pulse. The shape of the rising portion of the incident compressive pulse does not af- 7 feet the magnitude or the location of the peak tensile stress developed during the reflection. 1

The purpose of fragmenting the pavement is to facilitate its removal in excavation. Therefore, the range of particle size as well as the average size is important, as this determines the amount of interlocking of the particles.

The fragmentation process of pavement breaking by pulsed water jet impact is complex. A fracture will be initiated in the pavement when the stresses induced exceed its strength. The stresses may be either tensile, compressive, shear, or any combination of the three stress states. The fragmentations may, for convenience, be referred to as steps of cratering, radial tensile cracking, reflected tensile pulsing and, possibly, secondary wave propagations for radial cracking.

At the point of impact, a crushed zone or crater 82 is formed, as indicated in FlGS. 8 and 9. The limits of this hemispherically shaped shattered crater 82 are defined as that volume where the compressive/shear stress of the shock wave exceeds the compressive/shear strength of the pavement. The of the shock wave at any point depends on the impact pressure, P and the attenuation of the shock wave with the distance traveled.

Radiating from the crater 82 are a system of radial cracks 83, as seen in FIGS. 10. These cracks 83 are produced by tangential tensile stresses which result from the radial compression of the pavement due to the passing of the compressive shock wave. These cracks will propagate until the intensity of the tangential tensile stress falls below the strength of the pavement.

A third type of fragmentation occurs when a compressive shock wave is reflected as a tensile wave at the free surface 80. Where several free surfaces exist, the fragmentation pattern becomes quite complex. For purposes of explanation, only one free surface 80 is considered. The shock wave impinges on the free surface 80 and is reflected in the same way, as illustrated in FIGS. 7A-7F. It may be noted that the shock wave front extends from a to a in FIG. 11A, and that it impinges simultaneously over the entire free surface 80. The tensile stress builds up in the reflection process until it just exceeds the tensile strength of the pavement material, as illustrated in FIG. 118. At this point, the pavement cracks parallel to the free surface, as illustrated in FIG. 11C, at 85. The remaining compressive strain energy in the impinging pulse is transformed into tensile stress at the new surface, as illustrated in F [6. 11C and when it exceeds the strength of the pavement, a new slab is formed. This process continues to form cracks until the remaining strain energy is transformed into a tensile pulse which is below the tensile strength of the material. Fragmentation by this process then ceases. The number of such new free surfaces created is a function of the shape of the compressive pulse, the original free surface orientation, the ratio of pulse intensity to strength and the homogenity of the material, and the interference patterns produced by the reflection of pulses off various oriented primary and secondary free surfaces. These three type of fracturing occur within the first few milliseconds after impact.

There is also another stage of fragmentation which may occur. It is a slower process. The portion of the water pulse which impacts on the surface of the pavement after the above fragmentation process occurs enters the primary radial cracks and expands them. It also forces the pavement fragments at the free surfaces to yield and be displaced. When these frontal pieces yield, the back pressure is unloaded and the tension increases in the primary cracks which incline obliquely forward. These cracks then propagate to the closest free surface and complete the fragmenting of the pavement. This effect can be produced either by repeated impacts in the same crater with thin disclike water slugs or by single impacts with long cylindrical water slugs.

The combination of these stages and processes of fragmentation results in the complex breakage pattern expected in pavement breaking.

It will thus be seen that the the objects set forth above, among those made apparent from the preceding description, are efficiently attained and certain changes may be made in carrying out the above method. Accordingly, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

Now that the invention has been described, what is claimed as new and desired to be secured by Letters Patent is:

1. A method of fragmenting a section of pavement outlined by free surfaces to facilitate its subsequent removal comprising the steps of generating high-intensity, short-duration energy pulses which when impressed upon said pavement create compressive wave pulses therein, impressing upon said pavement said high-intensity short-duration energy pulses at a rate below BOO/per minute and in such a location and intensity that said compressive wave pulses which are created therein are reflected from said free surfaces outlining said pavement and produce tensile wave pulses which exceed the tensile strength of and cause the pavement to fracture.

2. The method of claim 1, wherein said high-intensity, shortduration energy pulses are of an intensity to create a fissure at the point of impact.

3. The method of claim 1, wherein said free surfaces are represented by discontinuities in said pavement.

4. The method of claim 3, wherein said discontinuities are fractures in said pavement.

5. The method of claim 1, wherein said high-intensity, shortduration energy pulses are impressed upon said pavement by means of high-intensity, pulsed, water jets.

6. The method of claim 1, wherein said free surfaces are formed by fracturing said pavement so as to define the area of pavement to be removed.

7. A method of fragmenting pavement provided with free surfaces to facilitate its subsequent removal comprising the step of forming free surfaces in the form of fractures defining an area of pavement to be removed by coupling microwave energy into said pavement to create therein at least two spaced apart heating patterns at a temperature such as to create stresses within said pavement at least between said heat patterns that exceed the strength of and thereby cause said pavement to fracture, and then imparting to said pavement high-intensity, short-duration compressive wave pulses of such number, location and intensity that tensile wave pulses which exceed the tensile strength of and cause the pavement to fracture are reflected from said free surfaces around or in said pavement.

8. The method of claim 7, wherein said high-intensity, shortduration compressive wave pulses are imparted to said area of pavement by means of high-intensity, pulsed, water jets. 

1. A method of fragmenting a section of pavement outlined by free surfaces to facilitate its subsequent removal comprising the steps of generating high-intensity, short-duration energy pulses which when impressed upon said pavement create compressive wave pulses therein, impressing upon said pavement said high-intensity short-duration energy pulses at a rate below 300/per minute and in such a location and intensity that said compressive wave pulses which are created therein are reflected from said free surfaces outlining said pavement and produce tensile wave pulses which exceed the tensile strength of and cause the pavement to fracture.
 2. The method of claim 1, wherein said high-intensity, short-duration energy pulses are of an intensity to create a fissure at the point of impact.
 3. The method of claim 1, wherein said free surfaces are represented by discontinuities in said pavement.
 4. The method of claim 3, wherein said discontinuities are fractures in said pavement.
 5. The method of claim 1, wherein said high-intensity, short-duration energy pulses are impressed upon said pavement by means of high-intensity, pulsed, water jets.
 6. The method of claim 1, wherein said free surfaces are formed by fracturing said pavement so as tO define the area of pavement to be removed.
 7. A method of fragmenting pavement provided with free surfaces to facilitate its subsequent removal comprising the step of forming free surfaces in the form of fractures defining an area of pavement to be removed by coupling microwave energy into said pavement to create therein at least two spaced apart heating patterns at a temperature such as to create stresses within said pavement at least between said heat patterns that exceed the strength of and thereby cause said pavement to fracture, and then imparting to said pavement high-intensity, short-duration compressive wave pulses of such number, location and intensity that tensile wave pulses which exceed the tensile strength of and cause the pavement to fracture are reflected from said free surfaces around or in said pavement.
 8. The method of claim 7, wherein said high-intensity, short-duration compressive wave pulses are imparted to said area of pavement by means of high-intensity, pulsed, water jets. 